linux-kernel - [PATCH tip/core/rcu 02/14] notifiers: Apply ACCESS

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Date:	Fri, 15 Nov 2013 16:40:05 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 02/14] notifiers: Apply ACCESS_ONCE() to avoid sparse false positive

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>

The sparse checking for rcu_assign_pointer() was recently upgraded
to reject non-__kernel address spaces.  This also rejects __rcu,
which is almost always the right thing to do.  However, the use in
notifier_chain_unregister() is legitimate: It is deleting an element
from an RCU-protected list, and all elements of this list are already
visible to caller.

This commit therefore silences this false positive by laundering the
pointer using ACCESS_ONCE() as suggested by Eric Dumazet and Josh
Triplett.

Reported-by: kbuild test robot <fengguang.wu@...el.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/notifier.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/notifier.c b/kernel/notifier.c
index 2d5cc4ccff7f..197eb70805a4 100644
--- a/kernel/notifier.c
+++ b/kernel/notifier.c
@@ -51,7 +51,8 @@ static int notifier_chain_unregister(struct notifier_block **nl,
 {
 	while ((*nl) != NULL) {
 		if ((*nl) == n) {
-			rcu_assign_pointer(*nl, n->next);
+			/* Both --rcu and visible, so ACCESS_ONCE() is OK. */
+			ACCESS_ONCE(*nl) = n->next;
 			return 0;
 		}
 		nl = &((*nl)->next);
-- 
1.8.1.5

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